166 Phenomena of Variation and GeograpJiical Distribution. 



selves, so as to cause the distinctions between the two forms to become 

 inconsiderable and indefinite, or where the differences, though constant, 

 are confined to one particular only, such as size, tint, or a single point 

 of difference in marking or in outhne, I class one of the forms as a var- 

 iety of the other. 



I find as a general rule tliat the constancy of species is in an inverse 

 ratio to their range. Those which are confined to one or two islands are 

 generally veryconstant. When they extend to many islands, consid- 

 erable variability appears ; and when they have an extensive range 

 over a large part of the archipelago, the amount of unstable variation 

 is very large. These facts are explicable on Mr. Darwin's principles. 

 When a si^ecies exists over a wide area, it must have had, and prob- 

 ably still possesses, great powers of dispersion. Under the different 

 conditions of existence in various portions of its area, different varia- 

 tions from the type woidd be selected, and, were they completely iso- 

 lated, would soon become distinctly modified forms ; but this process 

 is checked by the dispersive powers of the whole species which leads 

 to tlie more or less frequent intermixture of the incipient variety, 

 which thus become irregular and unstable. Where, however, a 

 species has a limited range, it indicates less active powers of dispersion, 

 and the process of modification under changed conditions is less inter- 

 fered with. The species will, therefore, exist under one or more per- 

 manent forms, according as portions of it have been isolated at a more 

 or less remote period. 



What is commonly called variation consists of several distinct phe- 

 nomena, which have been too often confounded. I shall proceed to 

 consider tliese under the heads of — 1st, simple variability ; 2d, poly- 

 morphism ; 3d, local forms; 4th, co-existing varieties ; 5 th, races or 

 subspecies ; and 6th, true species. 



1. Simple variahiliiy. — Under this head I include all those cases in 

 Avhich the sj)ecific form is to some extent unstable. Throughout the 

 whole range of the species, and even ni the progeny of individuals, 

 there occur continual and uncertain differences of form, analogous to 

 that variability which is so characteristic of domestic breeds. It is 

 impossible usefully to define any of these forms, because tliey are 

 indefinite gradations to each other form. Species which possess these 

 characteristics have always a wide range, and are more frequently the 

 inhabitants of continents than of islands, though such cases are always 

 excei)tional, it being far more common for specific foi-ms to be fixed 

 within very narrow limits of variation. The only good example of this 

 kind of variability which occurs among the Malayan Papilionidas is in 

 Fapilio Severm, a species inhabiting all the islands of the Moluccas and 



